Some day when Jesse Jackson says, as he so often does, "I'm leaving," some prominent Democrat may gently respond, "Why don't you, Jesse?"

The reason it has not happened so far is obvious. He is a splendid orator, with a following among the dispossessed and the aggrieved. White leaders fear to take him on lest they be charged with racism; besides, although their main problem is that they are seen as the prisoner of minorities and selfish special interests, they dare not offend blacks, 10 percent of the voters. Black leaders are mum because they fear backlash from their constituents.

Jackson talks darkly of a third party. It's just what the Democrats need. They are prostrate and bleeding, and their best hope is that they have hit bottom. Recent weeks have produced two ghastly catastrophes: an envenomed meeting of the Democratic National Committee, followed by a $100,000 filmed reply to the State of the Union address -- a "homemade movie" so hapless and groveling that it made Democrats groan and scream.

Jackson, hospitalized and bored, a lion in winter hungry for attention, chose the moment to renew his threat. He attacked the new party chairman, Paul G. Kirk Jr., declared that the Democrats were pandering to the white male vote and denounced the election of Roland Burris as vice chairman of the DNC. Burris beat out Richard G. Hatcher, Jackson's campaign chairman and the choice of the party's Black Caucus.

From his bed in Howard University Hospital, Jackson told Post reporter Juan Williams that he would not accord diplomatic recognition to Burris. He informed blacks that they must "reassess their loyalty to the Democratic Party."

Jackson has been reassessing his own loyalty since 1978, when he went before Republican National Committee members and promised that if they played ball with him, he would deliver blacks out of their "bondage" to the Democrats.

Jackson, a bolder politician than even Ronald Reagan, has never been modest about his ability to deliver votes. Throughout 1984, as a presidential candidate, he claimed that members of his Rainbow Coalition would not be able to decide between Reagan and Walter F. Mondale without his help. He didn't say that they would vote for Reagan, but he indicated that, absent a signal from him, they might stay home.

He made endless mischief for the Democrats in general and Mondale in particular. His demands were unceasing and elastic, and Mondale assigned one of his high command, Robert G. Beckel, full time to Jackson-handling. At one point, Mondale briefly revolted and in the hearing of several reporters declared he had had enough. But Beckel was dispatched to patch things up, and Jackson more or less rejoined the fold.

Much of last winter was given over to the embarrassment engendered by Jackson's friend and champion, Louis Farrakhan, whose anti-Semitism was virulent. Most of last February was dedicated to suspense: Would Jackson apologize to Jewish voters? Finally, on the eve of the New Hampshire primary, and after a fashion, he did.

The late spring was given over to new suspense: Would he disassociate himself from Farrakhan? At length he did, but not until he had accused Mondale of "bowing to Jewish leaders."

Once Mondale had won in the primaries, Jackson began a rain dance. Would he endorse Mondale? Ten days before the Democratic National Convention Jackson urged black voters "to await my signal and not automatically line up with the Democratic nominees."

Mondale, he said, needed his "voluntary, enthusiastic support to maximize the black vote."

"I'll play a trumpet with a clear sound," he promised.

At the convention, he made a spectacular speech. He asked delegates to vote for him on the first ballot.

It took him all summer to find the "clear sound." He had to be, he said, "the independent conscience of this party and the nation."

On Aug. 27, he declared that he was "prepared to bury the hatchet with Mondale." On Aug. 28, he "embraced" the Democratic ticket. Some weeks later he gave permission for "embraced" to be interpreted as "endorsed." Sens. Gary Hart (D-Colo.) and Edward M. Kennedy (D-Mass.) campaigned harder for Mondale.

Now he is restive again, demanding and threatening. And what can the Democrats do? Parley, capitulate, write him off? Or hope that he will come to agree with Beckel, who said, "I don't think the Rev. Jackson's enemy is Paul Kirk and the Democratic Party; it's Ronald Reagan and the radical right."